If I spill a small amount of water (or other similar liquid) on the worktop and leave it, it will eventually evaporate into vapour, as I'm sure everyone has seen. But the room is not at 100 degrees celsius (boiling point of water) so how can it evaporate?
by 
ChrisDG
ANSWERS: 3
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Water does not need to reach boiling point in order to evaporate. The molecules at the surface of water have more kinetic engergy than those underneath. The molecules escape into the surrounding atmosphere. The 100C boiling point is only applicable when the pressure is the equivalent of 1 Atmosphere or or 29.92 inches of mercury. Place some water in a vacuum chanber and it can boil at room temperature. ChrisDG: The Boiling Point is the maximum temperature a liquid can exist in certain situations. Water cannot exist as a liquid over 100 degrees celcius at 1 Atmosphere of pressure.
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All molecules vibrate; they hit each other. When water is ice, they vibrate just a little, because they are stuck together, and usually, before they vibrate very far in one direction, they run into other molecules which either stop them from leaving the ice's surface, or hold onto them with tiny electromagnetic forces. When you add heat, you make the impact of those vibrations greater, the places where they stick together are broken apart, and they often leave the surface of the water. That is evaporation and can happen at any temperature. At 100 degrees Centigrade, water not only evaporates from the top surface--it even evaporates from inside surfaces, and forms bubbles at the bottom of a pot. If you have a frost-free freezer you may have noticed that even ice can evaporate, although evaporating from the surface of a solid isn't called "evaporating," it's called "subliming."
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Imagine the liquid and the gas phase as two systems that have to be in balance. As long as the humidity in the air above the liquid phase is not saturated (i.e. 100% rel. humidity), liquid will evaporate. O.t.o.h if you have a dry surface and there is condensating humidity in the air, a liquid phase will condensate (first a film of moisture, dew, can grow into a puddle). If you hang out your wet, washed clothes for drying in Alaska at sub-zero temperature, they will eventually dry, because the air is very dry and will carry away the humidity from the clothes, although they are literally frozen! But what is boiling then? Above boiling temperature, the two phases stop to coexist, above boiling temperature there is no more liquid phase, only the gas phase.
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